In one type of known control system, a noise source transmits noise along a duct and the duct contains a sound control system comprising a microphone downstream from the noise source connected by way of a filter and a power amplifier to a loudspeaker which is itself downstream from the microphone. The loudspeaker generates sounds dependent on noise from the source with the aim of cancelling noise further down the duct and in particular at a certain point.
A major problem with a noise control system of this type is that it may become less effective as time goes on since, for example, analogue components drift and other conditions change as the layout of the system is altered. It is very inconvenient to continually stop the sound control system, re-measure the various characteristics involved in its construction and modify the system accordingly.